Replace NULL with nullptr in all C++ files.

webrtc/modules/desktop_capture/win/screen_capturer_win_gdi.cc

Index: webrtc/modules/desktop_capture/win/screen_capturer_win_gdi.cc
diff --git a/webrtc/modules/desktop_capture/win/screen_capturer_win_gdi.cc b/webrtc/modules/desktop_capture/win/screen_capturer_win_gdi.cc
index 87bc195ce40e06d0f0cfe0d71d9cf78bb523435f..87fb63b5dc8677ff31e0bbcd3e06f5afd9478a11 100644
--- a/webrtc/modules/desktop_capture/win/screen_capturer_win_gdi.cc
+++ b/webrtc/modules/desktop_capture/win/screen_capturer_win_gdi.cc
@@ -53,7 +53,7 @@ ScreenCapturerWinGdi::ScreenCapturerWinGdi(
 
 ScreenCapturerWinGdi::~ScreenCapturerWinGdi() {
   if (desktop_dc_)
-    ReleaseDC(NULL, desktop_dc_);
+    ReleaseDC(nullptr, desktop_dc_);
   if (memory_dc_)
     DeleteDC(memory_dc_);
 
@@ -128,7 +128,7 @@ void ScreenCapturerWinGdi::PrepareCaptureResources() {
   if (input_desktop && !desktop_.IsSame(*input_desktop)) {
     // Release GDI resources otherwise SetThreadDesktop will fail.
     if (desktop_dc_) {
-      ReleaseDC(NULL, desktop_dc_);
+      ReleaseDC(nullptr, desktop_dc_);
       desktop_dc_ = nullptr;
     }
 
@@ -157,7 +157,7 @@ void ScreenCapturerWinGdi::PrepareCaptureResources() {
       GetSystemMetrics(SM_CYVIRTUALSCREEN)));
   if (!screen_rect.equals(desktop_dc_rect_)) {
     if (desktop_dc_) {
-      ReleaseDC(NULL, desktop_dc_);
+      ReleaseDC(nullptr, desktop_dc_);
       desktop_dc_ = nullptr;
     }
     if (memory_dc_) {